The goal of this project is to determine the role of Synaptotagmin (Syt) IV in the control of mammalian synaptic transmission. The 15 identified Syts comprise a family of calcium (Ca) binding proteins that generally regulate membrane trafficking events. Syts I, II, and IV are found in synaptic vesicle membranes, which suggests that these isoforms are important for neurotransmission. Recent evidence reveals that Syts I and II are primary Ca sensors for neurotransmission, but there is little consensus on the role of Syt IV. The Syt IV protein is strongly elevated following seizures and has diminished Ca binding relative to Syt I and other isoforms. This proposal focuses on two Specific Aims to determine the function of Syt IV. Specific Aim 1 tests the hypothesis that Syt IV is a Ca sensor that is less effective than Syt I in promoting neurotransmitter release. Specific Aim 2 tests the hypothesis that elevation of Syt IV (as occurs following seizure) reduces neurotransmission. The experimental design will employ viral-mediated Syt IV expression and electrophysiological measurement of neurotransmission in cultured mammalian neurons. Evidence in support of these hypotheses would suggest a neuroprotective role for Syt IV following seizures.